A long-recognized need in the turbine engine art has been to attain higher operating temperatures in order to achieve a greater thermodynamic efficiency and an increased power output per unit of engine weight. Ideally, a turbine engine should operate with stoichiometric combustion in order to extract the greatest possible energy value from the fuel consumed. However, the temperatures resulting from stoichiometric and even near-stoichiometric combustion are beyond the endurance capabilities of metallic turbine engine components. Consequently, as the turbine engine art has progressed, an ever greater emphasis has been placed upon both enhanced cooling techniques and the development of temperature and oxidation resistant metals for use in components of the engine which are exposed to the highest temperatures. That is, cooling techniques and high temperature metals have been developed for each of combustion chambers, turbine stator nozzles, and turbine blades. This quest has led to the development of elaborate cooling schemes for all of these components as well as to classes of nickel-based "super alloy" metals which may be cast using directionally solidified or single crystal techniques. All in all, the quest for higher operating temperatures in a turbine engine fabricated of metallic components has led to a still increasing complexity and expense in the making of the engine.
An alternative approach to the attainment of higher operating temperatures in a turbine engine has been recognized. This approach involves the use of high-strength ceramic components in the engine. Ceramic components are better able than metals to withstand the high temperature oxidizing environment of a turbine engine. However, the term "high strength" in connection with ceramic structures must be viewed in context. While many ceramic materials exhibit superior high temperature strength and oxidation resistance, ceramics have historically been difficult to employ in turbine engines because of a comparatively low tensile fracture strength and a low defect tolerance. Consequently, a long-recognized need has been for the development of hybrid ceramic/metallic structures which utilize the characteristics of each material to best advantage in order to allow combustion in a turbine engine to take place closer to or at the stoichiometric level.
An additional problem with the use of ceramics in a turbine engine arises when the ceramic material is used to form a turbine disk structure. Particularly in an axial flow turbine having more than one turbine stages, the plural disks defining the stages of the turbine are formed as separate pieces. These disks then must be axially stacked and interconnected in torque transmitting relation while preserving coaxial alignment and axially spaced parallelism of the disks despite thermal and centrifugal cycling. Conventional metallic turbine structures employ a curvic coupling between adjacent turbine disks. With the development of ceramic materials and application of these materials to turbine disks the use of conventional curvic coupling structure was attempted. However the curvic coupling structure when fabricated of ceramic material consistently failed because of fracturing of the ceramic curvic teeth. Attempts to modify the conventional curvic structure to allow fabrication with ceramic material were unsuccessful.